In biology, melanins are heteropolymers consisting of L-dopa and its enzymatic derivatives. They are ubiquitous in living organisms and are produced throughout the zoological and botanical phyla. In mammalian skin, melanins are produced through enzymatic processes in specialized cells known as "melanocytes". Melanins are the pigments of mammalian skin and hair.
Mammalian melanins are highly insoluble and can be dissolved (solubilized) only through non-physiological treatments such as boiling in strong alkali, or through the use of strong oxidants such as hydrogen peroxide. Tyrosinase, a key enzyme in the melanin biosynthetic pathway, can catalyze the formation of melanin in a test tube using L-tyrosine, L-dopa or 5,6-dihydroxyindole as substrates; however, the product is an insoluble precipitate as described above.
Ito, "Reexamination of the Structure of Eumelanin", Biochimica et Biophysica Acta, 833, 155-161, 1986, mentions that natural melanin may be a polymer of 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid. Ito, however, does not teach or suggest combining these chemicals to form melanin.
Ito and Nicol, "Isolation of Oligimers of 5,6-Dihydroxyindole-2-carboxylic Acid from the Eye of the Catfish", Biochemical Journal, 143, 207-217, 1974, mention that oligimers of 5,6-dihydroxyindole-2-carboxylic acid exist in nature, for example in the tapetum lucidum of the sea catfish (Arius felis). Ito and Nicol, however do not teach or suggest that these structures could be used as a form of soluble melanin.
Palumbo, d'Ischia, Misuraca, and Prota, "Effect of metal ion on the rearrangement of dopachrome", Biochimica et Biophysica Acta, 925, 203-209, 1987, mention that the non-decarboxylative rearrangement of dopachrome at physiological pH values, leading mainly to the formation of 5,6-dihydroxyindole-2-carboxylic acid. They suggest that when considered in the light of the known metal accumulation in pigmented tissues, their results provide a new entry into the regulatory mechanisms involved in the biosynthesis of melanin. Palumbo et al., however, do not teach or suggest that such metal ions could be used to affect the color or formation of soluble melanin. Likewise, Leonard, Townsend, and King, "Function of Dopachrome Oxidoreductase and Metal Ions in Dopachrome Conversion in the Eumelanin Pathway", Biochemistry, 27, 6156-6159, 1988, present similar results to those of Palumbo et al. regarding metal ions and the formation of 5,6-dihydroxyindole-2-carboxylic acid from dopachrome. Like Palumbo et al., Leonard et al. also do not teach or suggest that such metal ions could be used to affect the color for formation of soluble melanin.
Korner and Pawelek, J. Invest Dermatol. 75, 192-195, 1980, report the presence of an enzymic activity, "dopachrome conversion factor", in melanocytes which catalyzes the conversion of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid. Pawelek, Biochem. Biophys. Res. Comm., 166, 1328-1333, 1990, showed that this conversion reaction is an isomerization and that dopachrome conversion factor is an isomerase. Aroca, Garcia-Borron, Solano, and Lozano, Biochem. Biophys. Res. Acta, 1035, 266-275, 1990, suggested that the mechanism is indeed an isomerization which involves a tautomeric shift and proposed that the enzyme be named "dopachrome tautomerase" with the official Enzyme Commission number of E.C.5.3.2.3. Tsukamoto, Jackson, Urabe, Motague, and Hearing, EMBO J., in press, 1992, reported identification of a gene for dopachrome tautomerase. The above information is reviewed by Pawelek, Pigment Cell Research, 4, 53-62, 1991.
Wolfram et al. (U.S. Pat. No. 4,968,497) disclose tanning by applying a composition comprising melanin precursors or melanin precursor-like materials to the skin. They do not teach or claim that applying melanins, either natural or synthetic, to the skin will mimic skin tanning. Rather they teach that such melanins achieve "only superficial external tanning results which is readily removed by rinsing with water or rubbing with a towel".
Herlihy (U.S. Pat. No. 4,515,773) discloses a skin tanning composition containing a melanin precursor and a tyrosinase enzyme in a cosmetic base. He does not, however, teach or claim the application of melanins, either natural or synthetic to the skin.
Gaskin (U.S. Pat. No. 4,806,344) discloses a sun protectant composition comprising "melanin" as one of the ingredients. Gaskin further discloses that the melanin is "synthesized from tyrosinase and DOPA". She does not teach or suggest that such melanin is synthesized non-enzymatically or that it is comprised of precursors which confer either negative or positive charges on the polymer, rendering it soluble in aqueous solutions.
Many reports exist exploring the role of sulfhydryl compounds such as cysteine or glutathione in determining the red or yellow colors in melanin (see review by Prota, d'Ischia, and Napolitano, "The Regulatory Role of Sulfhydryl Compounds in Melanogenesis", Pigment Cell Research, Supplement 1, 48-53, 1988. However, these reports do not teach or suggest that said sulfhydryl compounds could be used to influence the colors of synthetic soluble melanins.
It would be of commercial value to have synthetic varieties of melanin with different solubility, molecular size, and color characteristics. That is, it would be useful to be able to produce various melanins of predictable colors and physicochemical characteristics through specific changes in the synthetic reaction conditions. For example, it would be of value to produce a melanin that, at neutral pH, is soluble in aqueous solvents such as those used in skin creams and lotions, while at acid pH, is soluble in organic solvents such as those used in oil-based paints.
Such melanins could impart a natural-appearing tan to mammalian skin and hair. Such melanins would also be effective as sun-screens, since melanins are the chemicals in the skin which absorb ultraviolet radiation and thus provide protection from its harmful effects, such as premature skin aging and the occurrence of skin cancers.
Such melanins would be effective in treating post-inflammatory hypopigmentation due to eczema, acne, trauma, burns and psoriasis. Also as a cover-up for vitiligo and other disorders of hypopigmentation, soluble melanins would be quite effective.
Since melanins absorb light throughout the ultraviolet and visible spectra, they would also be effective as glass or plastic tinting agents for eye glasses, contact lenses, car windows, house windows, office buildings, etc. Since melanins exhibit brown, red-brown, and golden-brown colors, they would be effective food and beverage colorants for such things as beer, tea, coffee, potato chips, and the like.
Likewise, by absorbing in the ultraviolet range, such melanins would be effective agents in protecting industrial materials against damage from ultraviolet radiation. By "industrial material" or "industrial product" is meant, for example, truck and car tiers, paints, laminating materials, plastics, synthetic resins, and fabrics, particularly fabrics containing nylon, and like materials or products. It is well known that the rate of deterioration of paints, wood, plastics and rubber is dramatically increased by exposure to ultraviolet radiation.
In application Ser. No. 674,489, filed Mar. 25, 1991, now pending, and in its parent applications, Ser. No. 603,111 filed Oct. 25, 1990, now pending, and Ser. No. 525,944 filed May 18, 1990, now pending, there are disclosed novel soluble melanins and processes for their production.
One such process involves reaction of dopachrome with at least one enzyme, now known as dopachrome tautomerase, derived from biological cells or tissues which contain a pigmentary system. Advantageously, 5,6-dihydroxyindole, indole-5,6-quinone and/or melanochrome are also present.
If 5,6-dihydroxyindole-2-carboxylic acid is included in the reaction system, dopachrome and dopachrome tautomerase may be omitted and a soluble melanin will still be produced.
While the resulting soluble melanins are quite satisfactory in performance, there are certain limitations in their solubility over a broad pH range. Moreover, the colors of the soluble melanins are somewhat limited in scope.